Impedance matching between transmission lines and circuit components is important for optimum circuit performance. Transmission line impedance matching is especially important in a radio frequency (RF) transceiver. Impedance mismatch causes power loss due to signal reflections at the transmission line and circuit component interface, resulting in degraded signal to noise ratio. For optimum performance and maximum power transfer, the impedance across a power amplifier's (PA) output terminals should be power matched to the impedance of transmission lines from the PA to the antenna. Similarly, for optimum performance and low noise operation, the impedance across a low noise amplifier's (LNA) input terminals should be matched to the impedance of transmission lines leading to the LNA's input terminals.
FIG. 1 illustrates a system 100 using a conventional path switching technique to provide impedance matching to a PA 105 and a LNA 110 of a RF transceiver. In transmit mode, system 100 switches to a transmission path 115 that is specifically configured to provide an impedance value that is best suited for PA 105. Path 115 includes a balun circuit 125 and an impedance matching circuit 130. Impedance matching circuit 130 provides a fixed impedance across the output terminals of PA 105. Balun circuit 125 converts differential balanced signals from PA 105 into single-ended signals for transmission by the antenna.
In receive mode, system 100 switches to another transmission path 120. Transmission path 120 is specifically configured to match its impedance with the impedance of the input terminals of LNA 110 using an impedance matching circuit 140. Path 120 further includes a balun circuit 135 that converts single-ended RF signals to differential balanced signals. As shown in FIG. 1, system 100 is expensive and has a large footprint due to the number of components used.
FIG. 2 illustrates a system 200 for providing impedance matching to a PA and LNA of a transceiver. System 200 includes a PA 205, a LNA 210, an antenna 215, a band pass filter 220, a balun circuit 225, and an impedance matching circuit 230. In receive mode, RF signals are received by antenna 215. The received RF signals are then filtered to remove unwanted frequencies by filter 220. At this point, the RF signals are single-ended signals, which are converted into differential balanced signals using balun circuit 225. Balun circuit 225 is also used to convert differential balanced signals from PA 205 into single-ended signals for transmission by antenna 215, in transmit mode.
In system 200, impedance matching circuit 230 provides impedance matching to PA 205 and LNA 210. However, the impedance match provided by circuit 230 is fixed for both transmit and receive modes. Therefore, the impedance matching cannot be optimized for both PA 205 and LNA 210. Circuit 230 matches the impedance between nodes 227 and 237 using transmission lines or a plurality of capacitors and inductors. For further detail on an impedance matching system similar to system 200, see U.S. Pat. No. 6,735,418, “Antenna Interface”, to MacNally et al., which is incorporated by reference in its entirety.
System 200 is an improvement over system 100. However, for certain RF frequencies or under certain conditions, system 200 does not provide optimum impedance matching for both PA 205 and LNA 210. Accordingly, what is needed is a transceiver with an improved impedance matching system.